Sodium carbonate (Na2CO3), also called soda ash, is an important, high volume chemical produced in the United States and used in the manufacture of glass, chemicals, soaps and detergents, and aluminum, as well as in textile processing, petroleum refining and water treatment, among many other uses.
In the United States, almost all sodium carbonate is obtained from subterranean deposits of naturally-occurring trona ore. The largest known trona ore deposits in the United States are located in the Green River Basin in southwestern Wyoming, mostly in Sweetwater County, Wyoming, and are typically about 800 to 3000 feet below ground level.
Trona ore consists primarily (80-95 wt %) of sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), and lesser amounts of sodium chloride (NaCl), sodium sulfate (Na2SO4), organic matter, and insolubles such as clay and shales. A typical analysis of crude trona ore being mined at Green River, Wyo. is as follows:
ConstituentsWeight Percentsodium sesquicarbonate90sodium chloride (NaCl)0.1sodium sulfate (Na2SO4)0.02organic matter0.3insolubles (clay and shales)9.6
Trona ore may be recovered from subterranean deposits, for further processing into soda ash, by mechanical mining techniques or by any of several various solution mining methods. The Green River trona ore deposits are presently being commercially mined both by mechanical mining and by solution mining processes.
Mechanical mining, also called dry mining, is carried out underground in the ore beds by mining crews and includes room-and-pillar and long wall methods. Mechanical mining methods are relatively costly and leave unrecovered a significant fraction of the trona ore in the beds being mined, so solution mining processes present an economical alternative to mechanical mining.
Solution mining utilizes conventional well drilling technology and involves injecting water or other aqueous-based mining solvent, via a drilled well hole, into a deposit of trona ore; allowing the mining solvent to dissolve soluble ore; pumping the resulting mining solution (mine water) via a drilled well hole to the surface; and processing the mine water to recover dissolved ore values from the solution in the form of sodium carbonate or other related sodium-based chemicals. Solution mining methods may also be employed for recovering alkali values from depleted ore deposits that have previously been mechanically mined and abandoned.
An alkali solution from solution mining of a NaHCO3-containing ore deposit such as trona typically contains dissolved sodium carbonate and sodium bicarbonate, as well as dissolved organic and inorganic impurities solubilized from the ore deposit. The sodium carbonate values in such alkali solutions are normally recovered as soda ash by various crystallization processes, and the impurities present in the alkali solution are typically removed via a purge stream of crystallizer mother liquor, which is discarded.
Alkali solutions containing Na2CO3 and NaHCO3 values may obtained not only via solution mining of NaHCO3-containing subterranean ore deposits but also from surface alkali brine lakes or alkali waste ponds. Numerous processes for recovering sodium carbonate and/or sodium bicarbonate from such alkali solutions are disclosed in the prior art. Only a few of these prior art processes mention that sulfide, in addition to sulfates, may be present as a contaminant in the alkali solution.
U.S. Pat. No. 2,784,056 of Wiseman (Stauffer Chemical) issued Mar. 5, 1957 describes a process for recovering sodium bicarbonate from naturally-occurring complex brines located at Searles Lake, Calif. The '056 patent discloses that the brines contain sodium sulfide and that the sulfide is released as hydrogen sulfide during carbonation of such brines. U.S. Pat. No. 4,291,002 of Arnold et al. (Kerr-McGee) issued Sep. 22, 1981 describes a process for recovering soda ash from Searles Lake brines or from artificial brines such as a burkeite solution (Na2CO3 and Na2SO4 double salt), via carbonation of such brines to make a sodium bicarbonate intermediate, which is then recrystallized as sodium carbonate.
U.S. Pat. No. 4,869,882 of Dome et al. (General Chemical) issued Sep. 26, 1989 describes a process for recovering soda ash from waste or storage ponds associated with a soda ash manufacturing facility, via neutralization of the alkali waste water with lime to convert bicarbonate to carbonate, evaporation, and then crystallization of sodium carbonate decahydrate, which is recovered. The process is described as being useful for recovery of soda ash values from alkali waste water having a high sulfate content.
U.S. Pat. No. 5,262,134 of Frint et al. (FMC) issued Nov. 16, 1993 describes a process for recovering sodium carbonate values from mining liquor obtained from solution mining of subterranean trona ore deposits, via sequential crystallizations of sodium sesquicarbonate and sodium carbonate decahydrate, the latter then being recrystallized as sodium carbonate monohydrate. The Frint '134 patent contains a detailed description of various prior art trona ore solution mining techniques and of the “sesquicarbonate” and “monohydrate” soda ash recovery processes applicable to dry-mined trona ore, and those disclosures of U.S. Pat. No. 5,262,134 are hereby incorporated by reference into the present specification.
U.S. Pat. No. 5,575,922 of Green et al. (Solvay) issued Nov. 19, 1996 describes a process for treating mine water from an underground trona mine with caustic soda, to raise the pH of the mine water to 11.5 to 14, to convert NaHCO3 in the mine water into Na2CO3 and remove some of the impurities. Green et al. '922 notes that the mine water may also include impurities such as chlorides, sulfides, sulfites, sulfates, iron pyrite from shale insolubles and dissolved organic compounds. The patent also states that the caustic soda treatment, at preferred pH values above 12.5, significantly reduces sulfide odor in the mine water.
Numerous soda ash recovery processes have been described in the patent literature for treating alkali solutions obtained from solution mining, and many include a step of decomposing sodium bicarbonate in the alkali solution, with the concurrent evolution of gaseous carbon dioxide, to covert the bicarbonate into sodium carbonate.
U.S. Pat. No. 5,283,054 of Copenhafer et al. (FMC) issued Feb. 1, 1994 describes a process for recovering sodium carbonate from aqueous mining solution obtained from solution mining of subterranean trona deposits. The process first converts sodium bicarbonate present in the aqueous mining solution to sodium carbonate, via evaporation and CO2 stripping, followed by neutralization with lime to decompose residual sodium bicarbonate in the evaporated solution. An intermediate product, sodium carbonate decahydrate, is crystallized from the NaHCO3-depleted solution and recovered, then redissolved and recrystallized as sodium carbonate monohydrate. The soda ash recovery process of the Copenhafer '054 patent is sometimes referred to as the Evaporation, Lime, Decahydrate, Monohydrate (ELDM) process.
Other soda ash recovery processes, analogous to the ELDM process, have been described in subsequent patents for recovery sodium carbonate values from alkali solutions.
U.S. Pat. No. 5,766,270 of Neuman et al. (Tg Soda Ash) issued Jun. 16, 1998 and U.S. Pat. No. 5,955,043 of Neuman et al. (Tg Soda Ash) issued Sep. 21, 1999 each describe processes for recovering sodium carbonate from dilute solution mining brines. In Neuman et al. '270, the sodium bicarbonate content of the mining brine is first lowered, via steam stripping, followed by crystallization of sodium carbonate decahydrate. In Neuman et al. '043, the bicarbonate content of the mining brine is first lowered, via neutralization with caustic soda or dilution, followed by crystallization of sodium carbonate decahydrate. Residual bicarbonate in the decahydrate mother liquor is removed via steam stripping.
Other patents and published patent applications that describe soda ash recovery processes that utilize alkali solutions from solution mining or from dissolution of mined trona ore and that have a unit operation or step that involves conversion of bicarbonate to carbonate, e.g., via steam stripping, include U.S. Pat. No. 6,228,335 of Copenhafer et al. issued May 8, 2001; U.S. Pat. No. 6,576,206 of Copenhafer et al. issued Jun. 10, 2003; U.S. Pat. No. 6,589,497 of Smith issued Jul. 8, 2003; U.S. Pat. No. 7,645,435 of Braman et al. issued Jan. 12, 2010; and U.S. Patent Application Publication No. 2010/0066153 of Day et al. dated Mar. 18, 2010.
Although many of the above-noted references mention sulfates as being present in solution mining liquor, only a few mention the presence of sulfide and none of these describe treatment methods for the specific removal of sulfides.
Hydrogen peroxide has long been recognized as a useful oxidizing agent in industrial applications, and the patent and technical literature disclose such applications. Some of these are directed to abatement of sulfide as a pollutant in aqueous media and gas streams.
Kibbel et al., in “Hydrogen Peroxide for Industrial Pollution Control,” Industrial Wastes, November/December 1972, pp. 824-839, describe the use of hydrogen peroxide for treatment of sulfide pollutants in a variety of industrial wastes, including sulfide present in NaCl brines.
U.S. Pat. No. 3,705,098 of Shepherd et al. (FMC) issued Dec. 5, 1972 describes a process for controlling sulfide and hydrogen sulfide in sewage using hydrogen peroxide. U.S. Pat. No. 3,966,450 of O'Neill et al. (FMC) issued Jun. 29, 1976 describes a process for controlling odor in an animal waste slurry using hydrogen peroxide.
U.S. Pat. No. 3,717,698 of Ilardi et al. (FMC) issued Feb. 20, 1973 describes a process for removing organic impurities in the monohydrate soda ash process, using hydrogen peroxide activated with sodium persulfate. The hydrogen peroxide is employed in an amount sufficient to yield active oxygen equivalent stoichiometrically to the organic impurities in the solution, and the amount of sodium persulfate employed is from one-sixth to one-half of the weight of the hydrogen peroxide.
U.S. Pat. No. 4,163,044 of Woertz (Union Oil) issued Jul. 31, 1979 describes a process for treating H2S-containing geothermal steam obtained during a drilling operation to reduce the hydrogen sulfide content of the steam, by employing a gas absorption process that uses aqueous sodium hydroxide or calcium hydroxide as the aqueous alkaline absorption solution to produce H2S-depleted steam. Following the H2S-absorption step, at least a portion of the aqueous, alkaline solution containing absorbed H2S is treated with hydrogen peroxide to effect a reduction in the sulfide content of the solution. The treated alkaline solution is then heated and recycled for further use as alkaline absorption solution that is contacted with the H2S-containing steam.
U.S. Pat. No. 4,361,487 of Hills et al. (FMC) issued Nov. 30, 1982 describes a process for removing hydrogen sulfide from the condensate of spent steam in a geothermal power plant by oxidizing such hydrogen sulfide to elemental sulfur using a peroxygen compound, preferably hydrogen peroxide, in the presence of a vanadate catalyst under neutral to alkaline conditions. The vanadate catalyst, preferably aqueous sodium vanadate, is added after the steam condensate is adjusted to pH 7 or higher with an alkaline reagent, and the peroxygen is introduced after the vanadate catalyst has been added to the pH-adjusted condensate. The Hills et al. '487 patent states that the presence of the vanadate catalyst serves to increase the efficiency of the oxidation of hydrogen sulfide to elemental sulfur by hydrogen peroxide and that the hydrogen peroxide also serves to regenerate the vanadate ion.
U.S. Pat. No. 4,552,668 of Brown et al. (FMC) issued Nov. 12, 1985 describes the use of hydrogen peroxide with free radical scavengers in viscous (polymer-thickened) aqueous well treatment fluids intended for subterranean petroleum production operations, to react with sulfide in such fluids.
U.S. Pat. No. 4,574,076 of Castrantas (FMC) issued Mar. 4, 1986 describes the removal of hydrogen sulfide from geothermal steam before the latter is released into the atmosphere, using aqueous alkaline hydrogen peroxide injected into the steam gas stream to oxidize the hydrogen peroxide.
U.S. Pat. No. 4,839,154 of Allison et al. (Conoco) issued Jun. 13, 1989 describes a process for converting sulfide into innocuous sulfur species, in oil field-produced water, using an oxidizing agent such as chlorine, hypochlorite, hydrogen peroxide, chlorine dioxide sulfur dioxide or ozone in combination with a surface active agent.
U.S. Pat. No. 7,883,626 of Sharkey, Jr. et al. (Waterways Restoration) issued Feb. 8, 2011 describes the neutralization of acid mine drainage (e.g., from coal or metal mining operations) derived from the oxidation of sulfide minerals (e.g., iron sulfide in coal regions), exposed to water and oxygen, to form acidic, iron and sulfate-rich drainage. Sharkey, Jr. et al. '626 utilizes a multi-component formulation containing a neutralizer/binder component, a dissolution control/filtration component, an oxidizing agent and a dispersant/neutralizer component. The oxidizing component may be calcium peroxide, potassium permanganate, or hydrogen peroxide.
The present invention provides a method for the targeted and efficient oxidation of sulfides present in sulfide-containing alkali solutions using hydrogen peroxide to form soluble reaction products. The method is particularly useful for the removal of sulfide from NaHCO3-containing alkali solutions that are subsequently processed to convert such NaHCO3 into Na2CO3 via steam stripping or evaporation, since the sulfide would otherwise be volatilized as hydrogen sulfide (H2S) along with the gaseous CO2 byproduct of the bicarbonate decomposition reaction.